JP7716585B2 - Raw material for coating material for secondary battery separator, coating material for secondary battery separator, secondary battery separator and secondary battery - Google Patents

Description

The present invention relates to a coating material raw material for secondary battery separators, a coating material for secondary battery separators, a secondary battery separator, and a secondary battery. More specifically, the present invention relates to a coating material raw material for secondary battery separators, a coating material for secondary battery separators containing the coating material raw material for secondary battery separators, a secondary battery separator having a coating film of the coating material for secondary battery separators, and a secondary battery having the secondary battery separator.

Traditionally, secondary batteries are equipped with a separator that separates the positive and negative electrodes and allows ions in the electrolyte to pass through.

An example of such a separator is a polyolefin porous membrane.

On the other hand, a coating layer may be provided on the surface of the separator to impart heat resistance. Such a coating layer contains a binder resin and inorganic particles.

Another example of a binder resin is an acrylic water-soluble polymer. From the perspective of dispersibility, it is known to introduce reactive surfactant units into the acrylic water-soluble polymer.

As such a binder resin, for example, a water-soluble polymer containing reactive surfactant units has been proposed, in which the reactive surfactant units are 1 part by weight per 101 parts by weight of the water-soluble polymer (see, for example, Example 1 of Patent Document 1).

In addition, a lithium ion secondary battery binder composition has been proposed that contains, as a binder resin, a water-soluble polymer that includes reactive surfactant units and 20% by weight or more and 70% by weight or less of acid group-containing monomer units (see, for example, Patent Document 2).

JP 2014-222649 A International Publication No. 2014/185381 Pamphlet

However, the coating layer is required to have heat resistance, as well as breathability and adhesion to the substrate (specifically, adhesion to the polyolefin porous membrane).

On the other hand, Patent Document 1 has the disadvantage that the heat resistance, air permeability, and adhesion are reduced because the reactive surfactant units are too few compared to the water-soluble polymer.

Furthermore, Patent Document 2 has the disadvantage that the amount of acid group-containing monomer units is high compared to the water-soluble polymer, resulting in reduced heat resistance, air permeability, and adhesion.

The present invention provides a coating material raw material for secondary battery separators that has excellent heat resistance, air permeability, and adhesion, a coating material for secondary battery separators that contains the coating material raw material for secondary battery separators, a secondary battery separator that has a coating film of the coating material for secondary battery separators, and a secondary battery that has the secondary battery separator.

The present invention [1] relates to a coating material raw material for secondary battery separators, which comprises a water-soluble polymer, the water-soluble polymer comprising a first structural unit derived from a reactive surfactant and a second structural unit derived from an acidic group-containing vinyl monomer, the content of the first structural unit being 3% by mass or more and 30% by mass or less in the water-soluble polymer, and the content of the second structural unit being 3% by mass or more and 19% by mass or less in the water-soluble polymer.

The present invention [2] includes the coating material raw material for secondary battery separators described in [1] above, in which the glass transition temperature of the water-soluble polymer is 150°C or higher.

The present invention [3] includes the coating material raw material for secondary battery separators described in [1] or [2] above, in which the water-soluble polymer further contains a third structural unit derived from an amide group-containing vinyl monomer, and the content of the third structural unit in the water-soluble polymer is 51% by mass or more and 94% by mass or less.

The present invention [4] includes a coating material raw material for secondary battery separators described in any one of [1] to [3] above, in which the reactive surfactant is an ether sulfate-type anionic reactive surfactant.

The present invention [5] includes the coating material raw material for secondary battery separators described in [4] above, in which the ether sulfate-type anionic reactive surfactant is an ethylene oxide adduct.

The present invention [6] includes a coating material raw material for secondary battery separators described in any one of [1] to [3] above, in which the reactive surfactant is an ethylene oxide-added nonionic reactive surfactant.

The present invention [7] includes a coating material raw material for secondary battery separators described in [5] or [6] above, in which the number of moles of ethylene oxide added is 5 moles or more and 30 moles or less.

The present invention [8] includes a coating material for a secondary battery separator, which comprises the coating material raw material for a secondary battery separator described in any one of [1] to [7] above and inorganic particles.

The present invention [9] includes a secondary battery separator comprising a porous membrane and a coating film of the separator coating material described in [8] above, which is disposed on at least one side of the porous membrane.

The present invention [10] includes a secondary battery comprising a positive electrode, a negative electrode, and the secondary battery separator described in [9] above, disposed between the positive electrode and the negative electrode.

The coating material raw material for secondary battery separators of the present invention contains a water-soluble polymer containing a first structural unit derived from a reactive surfactant and a second structural unit derived from an acidic group-containing vinyl monomer in a specified ratio. Therefore, the coating material has excellent heat resistance, air permeability, and adhesion.

The coating material for secondary battery separators of the present invention contains the raw material for coating material for secondary battery separators of the present invention. Therefore, it has excellent heat resistance, air permeability, and adhesion.

The secondary battery separator of the present invention is provided with a coating film of the coating material for secondary battery separators of the present invention. Therefore, it has excellent heat resistance, air permeability, and adhesion.

The secondary battery of the present invention includes the secondary battery separator of the present invention, and therefore has excellent heat resistance, air permeability, and adhesion.

<Raw material for coating materials for secondary battery separators>
The coating material raw material for a secondary battery separator contains a water-soluble polymer.

The water-soluble polymer contains a first structural unit derived from a reactive surfactant and a second structural unit derived from an acidic group-containing vinyl monomer.

Such water-soluble polymers are polymers obtained by polymerizing water-soluble polymer raw materials.

A water-soluble polymer is defined as a polymer that has been dried, and when 1 g of the polymer is dissolved in 100 ml of water by stirring for 24 hours, and then filtered through a 300 mesh wire mesh, the residual solid content is 0.1% or less.

The water-soluble polymer raw material contains a reactive surfactant and an acidic group-containing vinyl monomer.

[Reactive surfactant]
The reactive surfactant is a surfactant that can be copolymerized with an acidic group-containing vinyl monomer. Specifically, the reactive surfactant is a surfactant that has an ethylenically unsaturated group (e.g., a vinyl group or a (meth)acryloyl group).

Such reactive surfactants include, for example, anionic reactive surfactants and nonionic reactive surfactants.

The anionic reactive surfactant is represented by, for example, the following formula (1):

In the above formula (1), ^{R 1} represents a hydrogen atom or a methyl group, and preferably represents a hydrogen atom.

In addition, in the above formula (1), ^R2 represents an alkylene group having 1 to 4 carbon atoms. That is, the anionic reactive surfactant represented by the above formula (1) is an alkylene oxide adduct. Examples of the alkylene group having 1 to 4 carbon atoms include a methylene group, an ethylene group, a propylene group, and a butylene group. ^R2 preferably represents an ethylene group from the viewpoint of improving heat resistance, air permeability, and adhesion (specifically, adhesion to a porous membrane (described later)). That is, the anionic reactive surfactant represented by the above formula (1) is preferably an ethylene oxide adduct.

In addition, in the above formula (1), ^R3 represents an alkyl group. Examples of the alkyl group include a linear alkyl group and a branched alkyl group. Examples of the linear alkyl group include a linear alkyl group having 1 to 20 carbon atoms. Examples of the linear alkyl group having 1 to 20 carbon atoms include a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group (a lauryl group), a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, and an eicodecyl group.

Examples of branched alkyl groups include branched alkyl groups having from 3 to 16 carbon atoms. Examples of branched alkyl groups having from 3 to 16 carbon atoms include isopropyl, isobutyl, s-butyl, t-butyl, 1-ethylpropyl, 1-methylbutyl, 1-methylpentyl, 1,1-dimethylpropyl, 1,1-dimethylbutyl, thexyl, cyclohexyl, 1,1-dimethylpentyl, 1-methylhexyl, 1,1-dimethylhexyl, 1-methylheptyl, 2-methylbutyl, 2-ethylbutyl, 2,2-dimethylpropyl, cyclohexylmethyl, 2-ethylhexyl, 2-propylpentyl, and 3-methylpentyl groups.

^R3 preferably represents a linear alkyl group. ^R3 more preferably represents a linear alkyl group having from 8 to 20 carbon atoms. ^R3 further preferably represents an undecyl group.

In addition, in the above formula (1), ^R4 represents an alkylene group. Examples of the alkylene group include a linear alkylene group and a branched alkylene group. Examples of the linear alkylene group include a linear alkylene group having 1 to 10 carbon atoms. Examples of the linear alkylene group having 1 to 10 carbon atoms include a methylene group, an ethylene group, a propylene group, a butylene group, a pentylene group, a hexylene group, a heptylene group, an octylene group, a nonylene group, and a decylene group.

Examples of branched alkylene groups include alkylene groups having 3 to 10 carbon atoms. Examples of alkylene groups having 3 to 10 carbon atoms include isopropylene, isobutylene, s-butylene, t-butylene, isopentylene, s-pentylene, 2-methylhexylene, and 2-ethylhexylene.

^R5 represents an anionic hydrophilic group. Examples of anionic hydrophilic groups include a carboxyl group, a sulfonic acid group, and a phosphate group. These functional groups may also form salts (e.g., sodium salts, potassium salts , calcium salts, magnesium salts, and ammonium salts). ^R5 preferably represents a salt of a sulfonic acid group (e.g., —SO ₃ X, where X represents a salt) from the viewpoint of improving heat resistance, air permeability, and adhesion. In other words, the anionic reactive surfactant is preferably an ether sulfate-type anionic reactive surfactant. ^R5 more preferably represents —SO ₃ NH ₄ .

m represents 0 or 1. m preferably represents 0.

n represents the number of moles of alkylene oxide (preferably ethylene oxide) added. n is, for example, 4 or more, and from the viewpoint of suppressing copolymerization difficulties of the reactive surfactant due to a decrease in the solubility of the reactive surfactant in water and improving heat resistance and adhesion, preferably 5 or more, more preferably 10 or more, and from the viewpoint of improving heat resistance and adhesion, for example, 45 or less, preferably 40 or less, more preferably 35 or less, and even more preferably 30 or less, particularly preferably 25 or less, and most preferably 20 or less.

The anionic reactive surfactant is preferably represented by the following formula (2) from the viewpoint of improving heat resistance, air permeability, and adhesion.

The anionic reactive surfactant represented by the above formula (2) is an anionic reactive surfactant in which, in the above formula (1), R ¹ represents a hydrogen atom, R ² represents an ethylene group, R ³ represents an undecyl group, R ⁵ represents —SO ₃ NH ₄ , and m represents 0.

Commercially available anionic reactive surfactants can also be used. Examples of such commercially available products include the ADEKA REASOAP SR series (anionic reactive surfactants represented by the above formula (2), manufactured by ADEKA Corporation).

The nonionic reactive surfactant is represented by, for example, the following formula (3):

In the above formula (3), R ¹ to R ⁴ , m and n have the same meanings as R ¹ to R ⁴ , m and n in the above formula (1).

In the above formula (3), R ¹ preferably represents a hydrogen atom.

In addition, in the above formula (3), ^R2 preferably represents an ethylene group. That is, the nonionic reactive surfactant is preferably an ethylene oxide adduct (an ethylene oxide adduct-type nonionic reactive surfactant) from the viewpoint of improving heat resistance, air permeability, and adhesion.

In the above formula (3), ^R3 preferably represents a linear alkyl group. ^R3 more preferably represents an undecyl group. In the above formula (3), ^R4 preferably represents a linear alkylene group. ^R4 more preferably represents an ethylene group. In the above formula (3), m represents 1.

In addition, in the above formula (3), n is, for example, 4 or more, and from the viewpoint of preventing the reactive surfactant from becoming difficult to copolymerize due to a decrease in the solubility of the reactive surfactant in water and improving heat resistance and adhesion, n is, for example, 45 or less, preferably 40 or less, more preferably 35 or less, and even more preferably 30 or less, particularly preferably 25 or less, and most preferably 20 or less, from the viewpoint of improving heat resistance.

The nonionic reactive surfactant is preferably represented by the following formula (4):

The nonionic reactive surfactant represented by the above formula (4) is a nonionic reactive surfactant in which, in the above formula (3), ^R1 represents a hydrogen atom, ^R2 represents an ethylene group, ^R3 represents an undecyl group, ^R4 represents an ethylene group, and m represents 1.

Commercially available nonionic reactive surfactants can also be used. Examples of such commercially available products include the ADEKA REASOAP ER series (nonionic reactive surfactants represented by the above formula (4), manufactured by ADEKA Corporation) and the AQUALON KH series (nonionic reactive surfactants represented by the above formula (4), manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.).

The reactive surfactant is selected appropriately depending on the type of porous membrane (described below) and the type of surface treatment to be applied to the porous membrane (described below).

Reactive surfactants can be used alone or in combination of two or more types.

The content of the reactive surfactant is, for example, 3% by mass or more, preferably 5% by mass or more from the viewpoint of further improving adhesion, and, for example, 30% by mass or less, preferably 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less, and particularly preferably 9% by mass or less, relative to the water-soluble polymer raw material.

[Acidic group-containing vinyl monomer]
Examples of the acidic group-containing vinyl monomer include a carboxyl group-containing vinyl monomer, a sulfonic acid group-containing vinyl monomer, and a phosphoric acid group-containing vinyl monomer.

Carboxy group-containing vinyl monomers are vinyl monomers that contain a carboxy group and are copolymerizable with reactive surfactants.

Examples of carboxyl group-containing vinyl monomers include monocarboxylic acids, dicarboxylic acids, and salts thereof. Examples of monocarboxylic acids include (meth)acrylic acid. Examples of dicarboxylic acids include itaconic acid, maleic acid, fumaric acid, itaconic anhydride, maleic anhydride, and fumaric anhydride. Note that (meth)acrylic refers to methacrylic and/ or acrylic.

Preferably, the carboxyl group-containing vinyl monomer is a monocarboxylic acid. More preferably, the carboxyl group-containing vinyl monomer is (meth)acrylic acid. Even more preferably, the carboxyl group-containing vinyl monomer is methacrylic acid.

Sulfonic acid group-containing vinyl monomers are vinyl monomers that contain sulfonic acid groups and are copolymerizable with reactive surfactants.

Examples of sulfonic acid group-containing vinyl monomers include allyl sulfonic acid, methallyl sulfonic acid, and acrylamido t-butyl sulfonic acid. Sulfonic acid group-containing vinyl monomers also include their salts. Examples of salts of sulfonic acid group-containing vinyl monomers include alkali metal salts (e.g., sodium salts, potassium salts) and ammonium salts. Specific examples include sodium allyl sulfonate, sodium methallyl sulfonate, and ammonium methallyl sulfonate.

Phosphate group-containing vinyl monomers are vinyl monomers that contain a phosphate group and are copolymerizable with reactive surfactants.

An example of a vinyl monomer containing a phosphoric acid group is 2- methacryloyloxyethyl acid phosphate.

From the standpoint of heat resistance, preferred examples of acidic group-containing vinyl monomers include carboxyl group-containing vinyl monomers.

Acidic group-containing vinyl monomers can be used alone or in combination of two or more types.

The content of the acidic group-containing vinyl monomer is, for example, 3% by mass or more, preferably 5% by mass or more, and, for example, 19% by mass or less, preferably 15% by mass or less, more preferably 10% by mass or less, relative to the water-soluble polymer raw material, from the viewpoint of further improving heat resistance and adhesion.

[Amide group-containing vinyl monomer]
The water-soluble polymer raw material preferably contains an amide group-containing vinyl monomer. As will be described in detail later, when the water-soluble polymer raw material contains an amide group-containing vinyl monomer, the water-soluble polymer contains a third constitutional unit derived from the amide group-containing vinyl monomer.

Amide group-containing vinyl monomers are vinyl monomers that contain an amide group and are copolymerizable with reactive surfactants and acid group-containing vinyl monomers.

Examples of amide group-containing vinyl monomers include methacrylamide and acrylamide.

A preferred example of an amide group-containing vinyl monomer is methacrylamide.

Amido group-containing vinyl monomers can be used alone or in combination of two or more types.

The content of the amide group-containing vinyl monomer relative to the water-soluble polymer raw material is, for example, 20% by mass or more, preferably 40% by mass or more, more preferably 45% by mass or more, and from the viewpoint of further improving heat resistance, air permeability, and adhesion, even more preferably 51% by mass or more, particularly preferably 60% by mass or more, most preferably 70% by mass or more, and for example, 94% by mass or less, preferably 80% by mass or less.

[Copolymerizable Monomer]
The water-soluble polymer raw material preferably contains a copolymerizable monomer. As will be described in detail later, when the water-soluble polymer raw material contains a copolymerizable monomer, the water-soluble polymer contains a fourth constitutional unit derived from the copolymerizable monomer.

The copolymerizable monomer is a monomer that can be copolymerized with a reactive surfactant, an acid group-containing vinyl monomer, and an amide group-containing vinyl monomer.

Examples of copolymerizable monomers include (meth)acrylic acid alkyl esters and functional group-containing vinyl monomers (excluding acidic group-containing vinyl monomers and amide group-containing vinyl monomers).
), vinyl esters, aromatic vinyl monomers, N-substituted unsaturated carboxylic acid amides, heterocyclic vinyl compounds, vinylidene halide compounds, α-olefins, dienes, and crosslinkable vinyl monomers.

Examples of (meth)acrylic acid alkyl esters include alkyl (meth)acrylates having an alkyl moiety with 1 to 12 carbon atoms. Examples of alkyl (meth)acrylates having an alkyl moiety with 1 to 12 carbon atoms include alkyl (meth)acrylates having an alkyl moiety with 1 to 4 carbon atoms and alkyl (meth)acrylates having an alkyl moiety with 5 to 12 carbon atoms. Examples of alkyl (meth)acrylates having an alkyl moiety with 1 to 4 carbon atoms include methyl (meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, and t-butyl (meth)acrylate. Examples of alkyl(meth)acrylates having an alkyl moiety with 5 to 12 carbon atoms include n-amyl(meth)acrylate, isoamyl(meth)acrylate, n-hexyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, octyl(meth)acrylate, decyl(meth)acrylate, dodecyl(meth)acrylate, and octadecyl(meth)acrylate.

As the (meth)acrylic acid alkyl ester, preferably, an alkyl (meth)acrylate having an alkyl portion having 1 to 4 carbon atoms is used.

Examples of functional group-containing vinyl monomers (excluding acidic group-containing vinyl monomers and amide group-containing vinyl monomers) include hydroxyl group-containing vinyl monomers, amino group-containing vinyl monomers, glycidyl group-containing vinyl monomers, cyano group-containing vinyl monomers, and acetoacetoxy group-containing vinyl monomers.

Examples of hydroxyl group-containing vinyl monomers include 2-hydroxyethyl (meth)acrylate and 2-hydroxypropyl (meth)acrylate. A preferred example of a hydroxyl group-containing vinyl monomer is 2-hydroxyethyl (meth)acrylate. A more preferred example of a hydroxyl group-containing vinyl monomer is 2-hydroxyethyl methacrylate.

Examples of amino group-containing vinyl monomers include 2-aminoethyl (meth)acrylate, 2-(N-methylamino)ethyl (meth)acrylate, and 2-(N,N-dimethylamino)ethyl (meth)acrylate.

Examples of glycidyl group-containing vinyl monomers include glycidyl (meth)acrylate.

Examples of cyano group-containing vinyl monomers include (meth)acrylonitrile.

Examples of acetoacetoxy group-containing vinyl monomers include acetoacetoxyethyl (meth)acrylate.

Examples of vinyl esters include vinyl acetate and vinyl propionate.

Aromatic vinyl monomers include, for example, styrene, α-methylstyrene, p-methylstyrene, vinyltoluene, and chlorostyrene.

An example of an N-substituted unsaturated carboxylic acid amide is N-methylol (meth)acrylamide. A preferred example of an N-substituted unsaturated carboxylic acid amide is N-methylol acrylamide.

Examples of heterocyclic vinyl compounds include vinylpyrrolidone.

Examples of vinylidene halide compounds include vinylidene chloride and vinylidene fluoride.

Examples of α-olefins include ethylene and propylene.

Examples of dienes include butadiene.

Examples of crosslinkable vinyl monomers include vinyl monomers containing two or more vinyl groups, such as methylenebis(meth)acrylamide, divinylbenzene, polyethylene glycol chain-containing di(meth)acrylate, trimethylolpropane tetraacrylate, pentaerythritol triacrylate, and pentaerythritol tetraacrylate.

Preferably, the copolymerizable monomer is a functional group-containing vinyl monomer. More preferably, the copolymerizable monomer is a hydroxyl group-containing vinyl monomer.

Copolymerizable monomers can be used alone or in combination of two or more types.

The content of the copolymerizable monomer is, for example, 1% by mass or more, preferably 5% by mass or more, and, for example, 60% by mass or less, preferably 40% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less, and particularly preferably 10% by mass or less, relative to the water-soluble polymer raw material.

[Method of producing a water-soluble polymer]
The water-soluble polymer can be obtained by polymerizing a water-soluble polymer raw material by a known method.

Specifically, for example, water-soluble polymer raw materials and a polymerization initiator are mixed with water, the water-soluble polymer raw materials are polymerized, and then aging is performed if necessary.

The polymerization initiator is not particularly limited, and examples thereof include water-soluble initiators, oil-soluble initiators, and redox initiators. Examples of water-soluble initiators include persulfates (e.g., ammonium persulfate, potassium persulfate), hydrogen peroxide, organic hydroperoxides, and 4,4'-azobis(4-cyanovaleric acid ) . Examples of oil-soluble initiators include benzoyl peroxide and azobisisobutyronitrile.

Preferably, the polymerization initiator is a water-soluble initiator, more preferably a persulfate, and even more preferably ammonium persulfate.

The proportion of the polymerization initiator is, for example, 0.01 parts by mass or more, preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, even more preferably 0.5 parts by mass or more, and for example, 3 parts by mass or less, preferably 1 part by mass or less, per 100 parts by mass of the water-soluble polymer raw material.

Polymerization initiators can be used alone or in combination of two or more types.

The polymerization temperature under atmospheric pressure is, for example, 30°C or higher, preferably 50°C or higher, and for example, 95°C or lower, preferably 85°C or lower. The polymerization time is, for example, 0.5 hours or higher, preferably 1.5 hours or higher, and for example, 20 hours or lower, preferably 10 hours or lower.

The aging time is, for example, 0.5 hours or more, preferably 1.5 hours or more, and, for example, 8 hours or less, preferably 6 hours or less.

In addition, in the above polymerization, known additives such as pH adjusters, metal ion sequestering agents (e.g., ethylenediaminetetraacetic acid and its salts), and molecular weight regulators (chain transfer agents) (e.g., mercaptans, low-molecular-weight halogen compounds) can be blended in appropriate proportions in order to improve production stability.

In addition, a neutralizing agent such as ammonia can be added before or after the above polymerization to adjust the pH to a range of 5 or more and 11 or less.

As a result, a water-soluble polymer is obtained as a polymer of the water-soluble polymer raw material in the form of an aqueous solution containing the water-soluble polymer.

The solids concentration of the aqueous solution containing the water-soluble polymer is, for example, 10% by mass or more, and, for example, 50% by mass or less.

As described above, the water-soluble polymer is obtained by polymerizing a water-soluble polymer raw material containing a reactive surfactant and an acidic group-containing vinyl monomer.

Therefore, the water-soluble polymer contains a first structural unit derived from a reactive surfactant and a second structural unit derived from an acidic group-containing vinyl monomer.

The content of the first structural unit in the water-soluble polymer is 3% by mass or more, preferably 5% by mass or more from the viewpoint of further improving adhesion, and 30% by mass or less, preferably 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less, and particularly preferably 9% by mass or less.

If the content ratio of the first structural unit is above the above lower limit, heat resistance, air permeability, and adhesion can be improved.

On the other hand, if the content ratio of the first structural unit is less than the above lower limit, heat resistance, air permeability, and adhesion will decrease.

Furthermore, if the content ratio of the first structural unit is below the above upper limit, heat resistance, air permeability, and adhesion can be improved.

On the other hand, if the content ratio of the first structural unit exceeds the above upper limit, heat resistance, air permeability, and adhesion will decrease.

The content ratio of the first constitutional unit can be calculated from the amount of reactive surfactant charged. In other words, the content ratio of the first constitutional unit is synonymous with the content ratio of the reactive surfactant relative to the water-soluble polymer raw material described above.

In addition, the content of the second structural unit in the water-soluble polymer is 3% by mass or more, preferably 5% by mass or more, and 19% by mass or less, preferably 15% by mass or less, more preferably 10% by mass or less, from the viewpoint of further improving heat resistance and adhesion.

If the content ratio of the second structural unit is above the above lower limit, heat resistance, air permeability, and adhesion can be improved.

On the other hand, if the content ratio of the second structural unit is less than the above lower limit, heat resistance, air permeability, and adhesion will decrease.

Furthermore, if the content ratio of the second structural unit is below the above upper limit, heat resistance, air permeability, and adhesion can be improved.

On the other hand, if the content ratio of the second structural unit exceeds the above upper limit, heat resistance, air permeability, and adhesion will decrease.

The content of the second structural unit can be calculated from the amount of the acidic group-containing vinyl monomer charged. In other words, the content of the second structural unit is synonymous with the content of the acidic group-containing vinyl monomer relative to the water-soluble polymer raw material described above.

Furthermore, in the water-soluble polymer, the mass ratio of the first structural unit to the second structural unit (first structural unit/second structural unit) is, for example, 0.1 or more, preferably 0.2 or more, more preferably 0.3 or more, and for example, 15.0 or less, preferably 10.0 or less, more preferably 7.0 or less, and even more preferably 5.0 or less.

If the above mass ratio is above the above lower limit and below the above upper limit, heat resistance, air permeability, and adhesion can be improved.

Furthermore, if the water-soluble polymer raw material contains an amide group-containing vinyl monomer, the water-soluble polymer contains a third structural unit derived from the amide group-containing vinyl monomer.

The content of the third structural unit in the water-soluble polymer is, for example, 20% by mass or more, preferably 40% by mass or more, more preferably 45% by mass or more, and even more preferably, from the viewpoint of further improving heat resistance, air permeability, and adhesion, 51% by mass or more, particularly preferably 60% by mass or more, most preferably 70% by mass or more, and, for example, 94% by mass or less, preferably 80% by mass or less.

If the content ratio of the third structural unit is above the above lower limit, heat resistance, air permeability, and adhesion can be improved.

Furthermore, if the content ratio of the third structural unit is below the above upper limit, heat resistance, air permeability, and adhesion can be improved.

The content of the third structural unit can be calculated from the amount of the amide group-containing vinyl monomer charged. In other words, the content of the third structural unit is synonymous with the content of the amide group-containing vinyl monomer relative to the water-soluble polymer raw material described above.

If the water-soluble polymer raw material contains a copolymerizable monomer, the water-soluble polymer contains a fourth structural unit derived from the copolymerizable monomer.

The content of the fourth structural unit in the water-soluble polymer is, for example, 1% by mass or more, preferably 5% by mass or more, and, for example, 60% by mass or less, preferably 40% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less, and particularly preferably 10% by mass or less.

If the content ratio of the fourth structural unit is above the above lower limit, heat resistance, air permeability, and adhesion can be improved.

Furthermore, if the content ratio of the fourth structural unit is below the above upper limit, heat resistance, air permeability, and adhesion can be improved.

The content ratio of the fourth structural unit can be calculated from the amount of copolymerizable monomer charged. In other words, the content ratio of the fourth structural unit is synonymous with the content ratio of the copolymerizable monomer relative to the water-soluble polymer raw material described above.

The water-soluble polymer preferably includes a first structural unit and a second structural unit, as well as a third structural unit and a fourth structural unit.

The glass transition temperature of the water-soluble polymer is, for example, 120°C or higher, and from the viewpoint of improving heat resistance, it is preferably 150°C or higher, more preferably 180°C or higher, even more preferably 200°C or higher, particularly preferably 210°C or higher, and most preferably 220°C or higher, and is, for example, 300°C or lower.

The above glass transition temperature can be measured, for example, using a Discovery DSC2500 manufactured by TA Instruments.

From the above, a coating material raw material for secondary battery separators containing a water-soluble polymer is obtained.

The coating material raw material for secondary battery separators contains a water-soluble polymer containing a first structural unit derived from a reactive surfactant and a second structural unit derived from an acidic group-containing vinyl monomer, each in a specific proportion. This results in excellent heat resistance, air permeability, and adhesion. This coating material raw material for secondary battery separators is particularly suitable for use in the production of coating materials for secondary battery separators.

<Coating material for secondary battery separators>
The coating material for a secondary battery separator contains a raw material for a coating material for a secondary battery separator and inorganic particles.

Inorganic particles include, for example, oxides, nitrides, carbides, sulfates, hydroxides, and potassium titanate. Oxides include, for example, alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide. Nitrides include, for example, silicon nitride, titanium nitride, and boron nitride. Carbides include, for example, silicon carbide and calcium carbonate. Sulfates include, for example, magnesium sulfate and aluminum sulfate. Hydroxides include, for example, aluminum hydroxide and aluminum oxide hydroxide. Silicates include, for example, talc, kaolinite, dickite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, silica sand, and glass. Hydroxides are preferred inorganic particles. More preferably, the inorganic particles include aluminum oxide hydroxide.

The average particle diameter of the inorganic particles is, for example, 0.1 μm or more, preferably 0.5 μm or more, and, for example, 5 μm or less, preferably 1 μm or less.

The average particle size of inorganic particles can be measured by creating a particle size distribution curve using a laser diffraction/scattering particle size distribution measuring device and calculating the particle size equivalent to 50% by mass.

Inorganic particles can be used alone or in combination of two or more types.

The blending ratio of inorganic particles is, for example, 1,000 parts by mass or more, and, for example, 10,000 parts by mass or less, preferably 5,000 parts by mass or less, per 100 parts by mass of the coating material raw material for secondary battery separators.

To produce a coating material for secondary battery separators, first, inorganic particles and, if necessary, a dispersant are mixed with water to prepare an inorganic particle dispersion. If a dispersant is mixed, the coating material for secondary battery separators contains the dispersant.

Examples of dispersants include ammonium polycarboxylate and sodium polycarboxylate. Preferred dispersants include ammonium polycarboxylate.

The blending ratio of the dispersant is, for example, 0.5 parts by mass or more, preferably 1 part by mass or more, and, for example, 10 parts by mass or less, preferably 5 parts by mass or less, per 100 parts by mass of inorganic particles.

Dispersants can be used alone or in combination of two or more types.

Next, the raw materials for the coating material for secondary battery separators are added to the inorganic particle dispersion and stirred.

The stirring method is not particularly limited, and examples include a ball mill, bead mill, planetary ball mill, vibrating ball mill, sand mill, colloid mill, attritor, roll mill, high-speed impeller dispersion, disperser, homogenizer, high-speed impact mill, ultrasonic dispersion, and stirring blade.

Furthermore, additives such as hydrophilic resins, wetting agents, antifoaming agents, and pH adjusters can be blended in appropriate proportions into the coating material for secondary battery separators as needed. In other words, the coating material for secondary battery separators contains additives as needed.

These additives can be used alone or in combination of two or more types.

Furthermore, as will be described in more detail below, this secondary battery separator coating material can improve heat resistance and adhesion without incorporating a wetting agent. Therefore, preferably, the secondary battery separator coating material does not incorporate a wetting agent. In other words, the secondary battery separator coating material preferably does not substantially contain a wetting agent. "The secondary battery separator coating material does not substantially contain a wetting agent" means that the wetting agent content is, for example, 0.1% by mass or less, preferably 0.01% by mass or less, and more preferably 0.001% by mass or less, relative to the secondary battery separator coating material.

This results in a coating material for secondary battery separators (aqueous dispersion of coating material for secondary battery separators).

The solids concentration of the aqueous dispersion of the coating material for secondary battery separators is, for example, 10% by mass or more, preferably 20% by mass or more, more preferably 30% by mass or more, and, for example, 50% by mass or less.

The coating material for a secondary battery separator contains the above-mentioned raw material for the coating material for a secondary battery separator. Therefore , the coating material for a secondary battery separator has excellent heat resistance, air permeability, and adhesion. Furthermore, such a coating material for a secondary battery separator is particularly suitable for use in the production of secondary battery separators.

<Secondary battery separator>
The secondary battery separator includes a porous membrane and a coating film of a coating material for a secondary battery separator that is disposed on at least one surface of the porous membrane.

[Porous membrane]
Examples of the porous membrane include a polyolefin porous membrane and an aromatic polyamide porous membrane. Examples of the polyolefin porous membrane include a polyethylene porous membrane and a polypropylene porous membrane. A preferred example of the porous membrane is a polyolefin porous membrane. The porous membrane may be surface-treated as needed. Examples of the surface treatment include corona treatment and plasma treatment.

The thickness of the porous membrane is, for example, 1 μm or more, preferably 5 μm or more, and, for example, 40 μm or less, preferably 20 μm or less.

[Coating film]
The coating film is a heat-resistant layer for imparting heat resistance to the porous film, and is made of a coating material for a secondary battery separator.

The thickness of the coating film is, for example, 1 μm or more, preferably 3 μm or more, and, for example, 10 μm or less, preferably 8 μm or less.

[Method for manufacturing secondary battery separator]
The method for manufacturing a secondary battery separator includes a first step of preparing a porous membrane, and a second step of applying a separator coating material to at least one surface of the porous membrane.

(1st step)
In the first step, a porous membrane is prepared.

(Second process)
In the second step, a coating material for a secondary battery separator (aqueous dispersion of a coating material for a secondary battery separator) is applied to at least one surface of the porous membrane, and then dried as necessary to obtain a coating film.

There are no particular restrictions on the application method, and examples include gravure coater method, small diameter gravure coater method, reverse roll coater method, transfer roll coater method, kiss coater method, dip coater method, microgravure coat method, knife coater method, air doctor coater method, blade coater method, rod coater method, squeeze coater method, cast coater method, die coater method, screen printing method, and spray application method.

The drying temperature is, for example, 40°C or higher and, for example, 80°C or lower.

This produces a secondary battery separator comprising a porous membrane and a coating film of the above-mentioned secondary battery separator coating material placed on at least one side of the porous membrane.

In the above explanation, a coating film of a coating material for a secondary battery separator is placed on at least one side of the porous membrane, but the above coating film can also be placed on both sides of the porous membrane.

This secondary battery separator has a coating film of the above-mentioned secondary battery separator coating material. Therefore, it has excellent heat resistance, air permeability, and adhesion. Furthermore, such a secondary battery separator is particularly suitable for use in the manufacture of secondary batteries.

<Secondary battery>
The secondary battery comprises a positive electrode, a negative electrode, the above-mentioned secondary battery separator disposed between the positive electrode and the negative electrode, and an electrolyte impregnated in the positive electrode, the negative electrode, and the above-mentioned secondary battery separator.

As the positive electrode, for example, a known electrode comprising a positive electrode current collector and a positive electrode active material laminated on the positive electrode current collector is used.

Examples of conductive materials for the positive electrode current collector include aluminum, titanium, stainless steel, nickel, baked carbon, conductive polymers, and conductive glass.

The positive electrode active material is not particularly limited, but examples include known positive electrode active materials such as lithium-containing transition metal oxides, lithium-containing phosphates, and lithium-containing sulfates.

These positive electrode active materials can be used alone or in combination of two or more types.

As the negative electrode, for example, a known electrode comprising a negative electrode current collector and a negative electrode active material laminated on the negative electrode current collector is used.

Examples of negative electrode current collectors include conductive materials such as copper and nickel.

The negative electrode active material is not particularly limited, but may be a carbon active material. Examples of carbon active materials include graphite, soft carbon, and hard carbon.

These negative electrode active materials can be used alone or in combination of two or more types.

When a lithium ion battery is used as the secondary battery, the electrolyte may be, for example, a solution in which lithium salt is dissolved in a carbonate compound such as ethylene carbonate (EC), propylene carbonate (PC), or ethyl methyl carbonate (EMC).

To manufacture a secondary battery, for example, a separator for the secondary battery is sandwiched between a positive electrode and a negative electrode, and these are housed in a battery casing (cell), and an electrolyte is injected into the battery casing.
In this way, a secondary battery can be obtained.

The above secondary battery is equipped with the above secondary battery separator, and therefore has excellent heat resistance, air permeability, and adhesion.

<Action and effect>
The coating material raw material for secondary battery separators contains a water-soluble polymer containing a first constitutional unit derived from a reactive surfactant and a second constitutional unit derived from an acidic group-containing vinyl monomer in a predetermined ratio, and therefore has excellent heat resistance, air permeability, and adhesion.

Specifically, from the perspective of improving heat resistance and adhesion, it is being considered to incorporate a wetting agent into a coating material for secondary battery separators containing the above-mentioned raw materials for the coating material for secondary battery separators.

This allows the secondary battery separator coating material to adhere more firmly to the surface of the porous membrane, improving heat resistance and adhesion. However, when a wetting agent is added, free wetting agent penetrates the porous membrane, reducing its air permeability.

In contrast, the coating material raw material for secondary battery separators contains a water-soluble polymer that contains a first structural unit derived from a reactive surfactant and a second structural unit derived from an acidic group-containing vinyl monomer, each in a predetermined proportion.

This allows the heat resistance and adhesion of the coating material for secondary battery separators to be improved without the need for a wetting agent. Furthermore, because no wetting agent is added, the reduction in air permeability caused by the free wetting agent described above can be suppressed, resulting in improved air permeability.

The coating material for secondary battery separators contains the above-mentioned raw materials for the coating material for secondary battery separators. Therefore, it has excellent heat resistance, air permeability, and adhesion.

The secondary battery separator is provided with a coating film of the above-mentioned secondary battery separator coating material. Therefore, it has excellent heat resistance, air permeability, and adhesion.

The secondary battery includes the secondary battery separator, and therefore has excellent heat resistance , air permeability, and adhesion.

Next, the present invention will be described based on examples and comparative examples, but the present invention is not limited to these examples. Note that "parts" and "%" are by mass unless otherwise specified. Furthermore, specific numerical values for blending ratios (content ratios), physical properties, parameters, etc. used in the following description can be substituted with the corresponding upper limit (a numerical value defined as "equal to or less than") or lower limit (a numerical value defined as "equal to or greater than") of the blending ratios (content ratios), physical properties, parameters, etc. described in the "Description of the Invention" above.

<Ingredient details>
Details of the active ingredients used in the following examples and comparative examples are shown below.
SR-10: ADEKA REASOAP SR-10, compound represented by the above formula (2) (n=10), number of moles of ethylene oxide added: 10, manufactured by ADEKA Corporation SR-20: ADEKA REASOAP SR-20, compound represented by the above formula (2) (n=20), number of moles of ethylene oxide added: 20, manufactured by ADEKA Corporation ER-10: ADEKA REASOAP ER-10, compound represented by the above formula (4) (n=10), number of moles of ethylene oxide added: 10, manufactured by ADEKA Corporation ER-20: ADEKA REASOAP ER-20, compound represented by the above formula (4) (n=20), number of moles of ethylene oxide added: 20, manufactured by ADEKA Corporation ER-30: ADEKA REASOAP ER-30, a compound represented by the above formula (4) (n=30), number of moles of ethylene oxide added: 30, manufactured by ADEKA Corporation; ER-40: Adeka Reasoap; ER-40, a compound represented by the above formula (4) (n=40), number of moles of ethylene oxide added: 40, manufactured by ADEKA Corporation; KH-05: Aqualon KH-05, a compound represented by the above formula (4) (n=5), number of moles of ethylene oxide added: 5, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.; Mam: methacrylic acid; Mac: methacrylamide; HEMA: 2-hydroxyethyl methacrylate

<Production of raw material for coating material for secondary battery separator, coating material for secondary battery separator , and secondary battery separator>
Example 1
[Production of coating material raw material for secondary battery separator]
A separable flask equipped with a stirrer and reflux condenser was charged with 392.0 parts by mass of distilled water, which was then purged with nitrogen gas and heated to 80°C. Next, 0.6 parts by mass of ammonium persulfate was added, and SR-10, Mac, Mam, and HEMA were dissolved in water so that the first to fourth structural units were in the proportions listed in Table 1. The solution was partially neutralized with aqueous ammonia and continuously added over 3 hours, and then maintained for another 3 hours to complete the polymerization. An appropriate amount of water was added to obtain an aqueous solution of a water-soluble polymer with a solids content of 15.0% by mass. The pH of the water-soluble polymer was 6.0. This produced a coating material raw material for a secondary battery separator containing a water-soluble polymer.

[Production of coating material for secondary battery separator]
An inorganic particle dispersion was prepared by uniformly dispersing 100 parts by mass of aluminum hydroxide oxide (manufactured by Taimei Chemical Industry Co., Ltd., boehmite Grade C06, particle size: 0.7 μm) as inorganic particles and 3.0 parts by mass (solid content equivalent) of an aqueous solution of ammonium polycarboxylate (manufactured by San Nopco Ltd., SN Dispersant 5468) as a dispersant in 110 parts by mass of water.

Next, the above-mentioned secondary battery separator coating material raw material was added to this inorganic particle dispersion in an amount equivalent to 5 parts by mass of solids, and water was added to make the solids content 40% by mass, followed by stirring for 15 minutes. This produced a secondary battery separator coating material.

[Manufacturing of secondary battery separator]
(1st step)
A polyolefin resin porous membrane was prepared as the porous membrane. Specifically, the polyolefin resin porous membrane was SW509C+ (film thickness 9.6 μm, porosity 40.6%, air permeability 158 g/100 ml, surface density 5.5 g/m ² , Changzhou Xingyuan New Energy Materials Co., Ltd.).
Next, the surface of the polyolefin resin porous film was cut into an A4 size.

(Second process)
A coating material for a secondary battery separator (aqueous dispersion of a coating material for a secondary battery separator) was applied to the surface (one side) of the polyolefin resin porous membrane using a wire bar. Then, the coating material was dried at 50°C. As a result, a coating film (thickness 5 μm) was formed on the surface (one side) of the polyolefin resin porous membrane. As a result, a secondary battery separator was produced.

Examples 2 to 20 and Comparative Examples 1 to 8
A coating material raw material for a secondary battery separator, a coating material for a secondary battery separator, and a secondary battery separator were produced based on the same procedure as in Example 1. However, the formulation of each component was changed so that the first to fourth constitutional units were in the proportions shown in Tables 1 to 3.

<Evaluation>
[Glass transition temperature of water-soluble polymer]
The glass transition temperatures of the water-soluble polymers of each Example and Comparative Example were measured. Specifically, a Discovery DSC2500 manufactured by TA Instruments was used under measurement condition 1: R.T./300°C (held for 1 minute)/-90°C (held for 3 minutes); temperature increase rate 50°C/min, temperature decrease rate 10°C/min, second temperature increase rate 10°C/min, under a nitrogen atmosphere. The results are shown in Tables 1 to 3.
[Heat resistance]
The secondary battery separators of each example and comparative example were cut into 5 cm x 5 cm pieces to serve as test specimens. After leaving these test specimens in an oven at 150°C for 1 hour, the lengths of each side were measured and the thermal shrinkage rate was calculated. Heat resistance was evaluated according to the following criteria. The results are shown in Tables 1 to 3.
(standard)
⊚: The heat shrinkage rate was less than 5%.
Good: The heat shrinkage rate was 5% or more and less than 20%.
Δ: The heat shrinkage rate was 20% or more and less than 50%.
×: The heat shrinkage rate was 50% or more.

[Air permeability]
The air permeability of the secondary battery separators of each example and each comparative example was measured in accordance with JIS-P-8117 using an Oken-type air permeability and smoothness tester manufactured by Asahi Seiko Co., Ltd. The lower the air permeability, the better the air permeability can be evaluated. The air permeability was evaluated according to the following criteria. The results are shown in Tables 1 to 3.
(standard)
⊚: Air permeability was less than 210 s/100 ml.
Good: The air permeability was 210 s/100 ml or more and less than 250 s/100 ml.
×: The air permeability was 250 s/100 ml or more.

[Adhesion]
The adhesion of the secondary battery separators of each Example and Comparative Example was measured. Specifically, the coating film on the secondary battery separator was rubbed with an eraser. The adhesion was evaluated according to the following criteria. The results are shown in Tables 1 to 3.
(standard)
⊚: No peeling occurred even after rubbing five times with a load of 400 g.
○: Peeled off after rubbing less than 5 times with a load of 400 g.
×: Peeled off when rubbed with a load of 100 g.

The above invention is provided as an exemplary embodiment of the present invention, but this is merely an example and should not be interpreted as limiting. Modifications of the present invention that are obvious to those skilled in the art are intended to be included within the scope of the following claims.

The coating material raw material for secondary battery separators, the coating material for secondary battery separators, and the secondary battery separators of the present invention can be suitably used, for example, in the manufacture of secondary batteries. The secondary batteries of the present invention can be suitably used, for example, in various devices and automobiles.

Claims (10)

comprising a water-soluble polymer,
the water-soluble polymer comprises a first constitutional unit derived from a reactive surfactant and a second constitutional unit derived from an acidic group-containing vinyl monomer,
the content of the first constitutional unit in the water-soluble polymer is 3% by mass or more and 30% by mass or less,
A coating material raw material for a secondary battery separator, wherein the content of the second constitutional unit in the water-soluble polymer is 3% by mass or more and 19% by mass or less. A coating material raw material for secondary battery separators as described in claim 1, wherein the water-soluble polymer has a glass transition temperature of 150°C or higher. the water-soluble polymer further contains a third constitutional unit derived from an amide group-containing vinyl monomer,
2 . The coating material raw material for a secondary battery separator according to claim 1 , wherein the content of the third constitutional unit in the water-soluble polymer is 51% by mass or more and 94% by mass or less. A coating material raw material for secondary battery separators as described in claim 1, wherein the reactive surfactant is an ether sulfate type anionic reactive surfactant. A coating material raw material for secondary battery separators as described in claim 4, wherein the ether sulfate type anionic reactive surfactant is an ethylene oxide adduct. A coating material raw material for secondary battery separators as described in claim 1, wherein the reactive surfactant is an ethylene oxide adduct type nonionic reactive surfactant. A coating material raw material for secondary battery separators as described in claim 5, wherein the number of moles of ethylene oxide added is 5 moles or more and 30 moles or less. A coating material for a secondary battery separator, comprising the coating material raw material for a secondary battery separator according to claim 1 and inorganic particles. A porous membrane;
A secondary battery separator comprising: a coating film of the separator coating material according to claim 8 disposed on at least one surface of the porous membrane. A secondary battery comprising a positive electrode, a negative electrode, and the secondary battery separator described in claim 9 disposed between the positive electrode and the negative electrode.